Organic light-emitting devices (hereinafter referred to as “OLEDs”) containing a phosphorescent metal complex as a luminescent material have luminous efficiency markedly higher than that of light-emitting devices containing a fluorescent material. In view of this, the OLEDs attract much attention.
Among heavy-metal complexes such as platinum, osmium, and iridium, which can be used as the luminescent material, the iridium complex has the highest luminous efficiency.
However, in order to obtain high-performance OLEDs, an iridium complex (luminescent dopant) should be doped on a base material of a luminescent host such as 4,4′-N,N′-dicarbazole-biphenyl (hereinafter referred to as “CBP”) or the like while the iridium complex is precisely controlled at a low concentration within a certain concentration range (for example, 6% by weight to 10% by weight).
That is, the production of the high-performance OLEDs requires careful control of production steps to optimize a doping amount of the luminescent dopant. Further, there is a problem that prolonged use of such OLEDs causes phase separation between the luminescent host and the luminescent dopant.
In view of this, instead of this type of OLEDs (hereinafter referred to as “doped OLEDs”) in which the luminescent dopant is doped on the luminescent host, another type of OLEDs (hereinafter referred to as “non-doped OLEDs”) in which the luminescent dopant is not doped on the luminescent host is being developed. If non-doped OLEDs that practically exhibit sufficient performance can be produced, it is unnecessary to take account of such problems as the precise control of a doping amount and the phase separation.
Even if the non-doped OLEDs are produced by use of a conventional luminescent dopant, the performance of the non-doped OLEDs is at least about one order of magnitude lower than best-performance doped OLEDs in terms of obtainable luminance and luminous efficiency (see Non-patent Literature 1). This problem is presumably attributable to at least one of the following common characteristics among phosphorescent materials: (i) carrier (charge) transfer is extremely poor; and (ii) self-quenching is caused.
In view of this, as reported in Non-patent Literatures 2 to 5, novel phosphorescent materials suitable for the non-doped OLEDs are been developed.
Further, as reported in Non-patent Literature 6, an attempt to decrease self-quenching by introducing a stereocontrol spacer into a phosphorescent material is being also made.